The present invention relates to an ultrasonic flaw detection technique for inspecting a solid body, and in particular, to an ultrasonic flaw detection technique for carrying out ultrasonic flaw detection by a phased array technique by use of an array sensor.
As a nondestructive inspection technique for a solid body allowing propagation of both longitudinal waves and shear waves (e.g. steel), a technique using ultrasonic waves (ultrasonic flaw detection) has been generally used. As a type of the ultrasonic flaw detection, there exists flaw detection using the so-called phased array technique.
Here, the phased array technique is also called an “electronic scan technique”, in which a probe including a plurality of ultrasonic generator elements (made of piezoelectric elements) arranged in an array (the so-called “array probe”) is used. In the technique, electric signals as triggers for the generation of ultrasonic waves are successively supplied to the elements of the array probe at prescribed time intervals (delays) and the ultrasonic waves generated by the elements are superposed on one another to form a superposed wave, by which various conditions such as the transmission/reception angle/position of ultrasonic waves to/from the specimen being inspected, positions having enhanced energy due to the interference in the superposed wave (i.e. focal positions), etc. can be changed at high speed by means of electronic control.
The array probe is used for electrically scanning the flaw detection conditions since the transmission/reception angle/position and the focal positions of ultrasonic waves can be changed freely across a wide inspection range, by which an angle, position and focal points allowing reception of stronger reflected waves (echoes) from a reflector (defect, etc.) existing inside or on a surface of the specimen can be selected and thereby defects as reflectors can be found easily.
On the other hand, in a widely employed ultrasonic flaw detection technique using only one ultrasonic probe (two probes (transmission/reception probes) when separate probes are used for transmission and reception respectively), one probe can realize only one probe condition (transmission/reception angle, transmission/reception position, focal position), and thus a plurality of probes have to be prepared in order to achieve different flaw detection conditions.
Even the aforementioned phased array technique using an array probe is being adopted, in most cases, for the purpose of expanding the functions of conventional probes. Therefore, even when the integrity of a specimen is evaluated by use of the phased array technique, the so-called angled flaw detection technique (evaluating the integrity by letting shear waves or longitudinal waves propagate in the specimen in an oblique direction and receiving waves reflected by a reflector such as a defect) is mainly used, similarly to the case of flaw detection using a conventional probe.
The angled flaw detection technique can be characterized as having a common and fixed propagation mode (longitudinal or shear) of the wave transmitted, the wave propagating in the specimen and the wave received. For example, in an angled longitudinal wave flaw detection technique, a longitudinal wave transmitted is reflected by a reflector (defect, etc.) and the reflected wave is received by the probe also as a longitudinal wave.
Meanwhile, as a flaw detection technique using a fixed angle besides the angled flaw detection technique, there exist an ID creeping technique for judging whether there exists a reflector such as a defect and a mode conversion technique capable of roughly evaluating the dimensions of a defect.
These techniques can contribute to the improvement of reliability of the angled longitudinal wave technique. For example, when a defect existing in an inspection area of the specimen is searched for by use of an angled beam, there are cases where an echo (reflected wave) from a deformed part of the specimen (e.g. deformation caused by welding or machining) is received. In such cases, the discrimination between an echo caused by such a deformed part and an echo from a defect can be very difficult.
In such cases, if the aforementioned ID creeping waves or mode conversion waves are used together with the angled longitudinal waves (angled beam), discriminability of echoes can be increased and that contributes to the improvement of reliability of flaw detection results obtained by the angled flaw detection technique.
By the way, in the ID creeping waves and the mode conversion waves used in the aforementioned techniques, the wave transmitted, the wave propagating in the specimen and the wave received do not have the same propagation mode, differently from the case of the angled flaw detection technique. For example, in the ID creeping technique, shear ultrasonic waves (angle: approximately 30°), generated simultaneously with longitudinal waves (angle: approximately 70°) by a longitudinal wave probe, are used, and the propagation mode changes later as will be explained below.
Here, a brief outline of the propagation of ultrasonic waves in the ID creeping technique will be explained referring to FIG. 4. When a shear wave 401 is emitted from an ultrasonic transducer, the propagation mode of the wave changes from the shear wave to a longitudinal wave 402 (mode conversion) when the wave is reflected by a far surface (base) of the specimen. Thereafter, the longitudinal wave 402 is reflected by a crack corner of a crack (defect) 403. The crack corner of the crack is a portion near a surface of the specimen.
A longitudinal wave 404 (the longitudinal wave 402 after being reflected by the crack corner) propagates in the vicinity of the far surface of the specimen. During the propagation along the far surface, the longitudinal wave 404 converts into a shear wave 405 (mode conversion), by which the shear wave 405 returns to the ultrasonic transducer and is received as an echo from the crack corner.
As above, the ID creeping technique, enabling the reception of echoes from crack corners, is effective for judging whether a specimen has a defect or not.
In the so-called mode conversion technique using the mode conversion of ultrasonic waves as above, shear ultrasonic waves (angle: approximately 28°) generated simultaneously with longitudinal waves (angle: approximately 60°) by a longitudinal ultrasonic transducer are used.
Thus, a brief outline of the propagation of ultrasonic waves in the so-called mode conversion technique will be explained below referring to FIGS. 5A and 5B. The propagation mode of a shear wave 501 generated by an ultrasonic transducer changes from the shear wave to a longitudinal wave 502 when the wave is reflected by the far surface of the specimen.
In cases where a reflector 502 shown in FIG. 5A is a defect having a certain height, reflection occurs at the tip of the defect or on the surface of the defect on the way to the tip. A longitudinal wave 504 reflected by the defect returns directly to the ultrasonic transducer through the specimen and is received as an echo from the defect.
However, in the case of a reflector 507 shown in FIG. 5B which is relatively low, a longitudinal wave 506 (generated by the mode conversion from a shear wave 505 at the far surface of the specimen) can not meet the tip of the defect, by which there appears no ultrasonic wave returning to the ultrasonic transducer. As above, in the mode conversion technique, whether the defect has a certain height (approximately ⅓ of the wall thickness of the specimen) or not corresponds to the presence/absence of an echo from the defect. Thus, the mode conversion technique is effective for roughly determining the height of a defect.
However, since the two techniques explained above employ a judgment based on a waveform called “A-scan” (plotted on a graph with the vertical axis representing reception intensity of ultrasonic waves and the horizontal axis representing propagation distance or propagation time inside the specimen), it is extremely difficult and requires skill to clarify the origin of the complex propagation path inside the specimen shown in FIGS. 4, 5A and 5B and determine the presence/absence of a defect or the approximate size of the defect.
Meanwhile, in order to implement the aforementioned ID creeping technique or the mode conversion technique by use of an array probe, the array probe is required to generate both longitudinal waves and shear waves in intended directions.
However, with conventional array probes, the generation of longitudinal and shear waves in intended directions is generally accompanied by ultrasonic waves being radiated in other directions, by which the identification of propagation paths of received ultrasonic waves and the implementation of the above techniques by use of an array probe become difficult.
In the conventional phased array technique, two types of probes: an array probe making direct contact with the specimen for generating longitudinal waves (the so-called “array probe in contact technique”) and an array probe supporting both longitudinal waves and shear waves using a wedge shaped intermediate medium called “wedge” or “shoe” (the so-called “array probe with a wedge”) have been used mainly. Therefore, features and problems with each of the array probes will be explained below.
The array probe in contact technique is an array probe for longitudinal waves, placed to directly contact the specimen or to be in parallel with the specimen. In the array probe, ultrasonic transducer elements such as piezoelectric elements are arranged in a line (array) and the angle of transmission/reception of ultrasonic waves propagating in the specimen is electronically changed from vertical (angle: 0°) to 45° (or 60°).
In this case, longitudinal waves including components perpendicular to the specimen are generated by each ultrasonic transducer element of the array probe. Therefore, longitudinal waves propagating in an intended angle θ can be synthesized by giving a proper delay time determined by the following expression (1) to each element (see “Handbook of Ultrasonic Diagnostic Equipment (Revised Edition)”, pp. 39–40, Electronic Industries Association of Japan (1997), for example):τi=(i−1)P·sin θ/c  (1)where “i” denotes a serial number of each element, “τi” denotes a delay time given to the i-th element, “c” denotes wave velocity (propagation speed) of longitudinal waves in the specimen (solid body), “P” denotes the element pitch, and “θ” denotes the incident angle (incident direction) of the ultrasonic waves.
It is well known that ultrasonic waves propagating in other directions φ are also synthesized in addition to the ultrasonic waves (main beam) propagating in the intended direction θ.
For preventing the synthesis of the undesired ultrasonic waves (hereinafter referred to as “grating lobes”) other than the main beam propagating in the intended direction (angle) θ, the element pitch P in the expression (1) has to be set smaller than or equal to a value determined by the following expression (2):P=λ/(1+|sin θ|)  (2)where “λ” denotes the wavelength of longitudinal waves in the specimen (solid body).
Since the maximum value of the incident angle θ of the ultrasonic waves is 90°, the minimum value of the element gap in the expression (2) is ½ of the wavelength.
While the array probe in contact technique is capable of transmitting longitudinal waves in a wide range of angles without causing the grating lobes, a grating occurs to shear waves at the same time, hampering the implementation of the aforementioned ID creeping technique or mode conversion technique by use of the array probe.
There have been proposed a method and a device implementing an angled shear wave flaw detection technique by use of an array probe by focusing attention on shear waves simultaneously generated by the piezoelectric elements of the array probe, treating longitudinal waves generated simultaneously with the shear waves as noise, and reducing the noise (see JP-A-2001-255308, for example).
However, even the above proposition discloses nothing about an ID creeping technique or mode conversion technique that utilizes both longitudinal waves and shear waves simultaneously generated by the array probe.
Meanwhile, the array probe with a wedge is an array probe including an array sensor placed with a tilt angle relative to the specimen and an extra medium sandwiched between the array sensor and the specimen. Typical examples of the medium placed between the array sensor and the specimen include water and synthetic resin (acrylic, polystyrene, polyimide, etc.). The intermediate medium is called a “wedge” or “shoe” as mentioned above.
By use of the wedge, even when the incident angle of the ultrasonic waves upon the wedge is small, a large refractive angle for the incidence upon the specimen can be achieved thanks to the refraction of the ultrasonic waves (see “Ultrasonic Testing (Revised Edition)”, pp. 35–47 and 746, The 19th Committee on Steelmaking, Japan Society for the Promotion of Science (1974), for example).
The following equation (3) represents the relationship between the incident angle θ′ upon the wedge and the refractive angle θ into the specimen:θ′=sin−1 θ(sin θ×V′/V)  (3)where “θ′” denotes the incident angle of the longitudinal waves upon the wedge, “θ” denotes the refractive angle of the ultrasonic waves incident upon the specimen, “V′” denotes wave velocity of longitudinal waves in the wedge, and “V” denotes wave velocity of longitudinal waves in the specimen (solid body).
For example, when ultrasonic waves are incident upon steel (iron) (wave velocity: approximately 5900 m/s (longitudinal wave), approximately 3000 m/s (shear wave)) from water (wave velocity: approximately 1500 m/s), shear waves incident upon the specimen at an incident angle of 70° can be achieved by letting the ultrasonic waves incident upon the water at an incident angle of approximately 14°. However, shear waves at an angle of approximately 29° develop in the steel at the same time.
By this, multiple reflection echoes inside the wedge are received by the probe as noise signals, which can hamper the identification of echoes from defects.